SIZE DISTRIBUTION OF PARTICLES IN SATURN'S RINGS
University of Leicester
An international team of scientists has discovered that planetary
rings, such as those orbiting Saturn, have a universally similar
particle distribution. The study also suggests that Saturn's rings
are in a steady state that does not depend on their history. Saturn's
rings are relatively well studied and it is known that they consist of
ice particles ranging in size from centimetres to about ten metres.
The particles are probably the relics of some catastrophic event in
the far past, and it is not surprising that there exist debris of all
sizes, varying from very small to house-size pieces. What is
surprising is that the relative abundance of particles of different
sizes follows, with a high accuracy, a mathematical law of 'inverse
cubes'. That is, the abundance of 2-metre-size particles is 8 times
smaller than the abundance of 1-metre particles, the abundance of
3-metre particles is 27 times smaller and so on. That holds true up
to the size of about 10 metres, then follows an abrupt drop in the
abundance of particles. The reason for that drop, as well as the
reason for the inverse-cube law, has been unknown, but the team
considers that it has now understood the particle-size distribution.
In particular, the study shows that the observed distribution is not
peculiar to Saturn's rings, but has a universal character; in other
words, it is expected to be generic for all planetary rings which
consist of particles of a similar nature.
Most of the planets in the Solar System have smaller bodies
(satellites), that orbit them. Some of them, such as Saturn, Jupiter,
Uranus and Neptune, additionally possess planetary rings -- a
collection of still smaller bodies of different sizes that also orbit
a planet. It is likely that planetary rings also exist beyond the
Solar System. Certain large asteroids, such as Chariklo and Chiron,
only a few hundred kilometres in diameter, are also surrounded by
rings. The rather general mathematical model elaborated in the study
with the focus on Saturn's rings may be applicable to other systems,
where particles merge, colliding with slow velocities and breaking
into small pieces that then collide with larger impact speeds. Such
systems exist in nature and in industry, and should exhibit the law
of inverse cubes and then a drop in large-particle abundance in their
particle-size distribution.
EXCEPTIONAL PLANETARY SYSTEM IN CASSIOPEIA
Universite de Geneve
Astronomers have found a planetary system in Cassiopeia, only about
6.5 parsecs (21 light-years) away from us. The star HR 8832 hosts one
outer giant planet and three inner super-Earths, one of which transits
in front of the star. The transiting super-Earth has a density
similar to the Earth's. It is by far the closest transiting planet
known today. The star, a 5.5-magnitude K dwarf, slightly cooler and
less massive than the Sun, is bright enough to be seen with the naked
eye under dark skies; it is circumpolar as seen from north-European
latitudes. When the first HARPS-N radial-velocity measurements
indicated the presence of a 3-day planet around HR 8832, astronomers
immediately asked NASA for observing time on the Spitzer space
telescope. The idea was to check for a possible transit of the planet
in front of the star, that would enable the size of the planet to be
determined. The mass of the planet obtained from the ground-based
radial velocities, combined with the planet radius derived from space
observations with Spitzer, yield the mean density of the planet.
HR 8832 b is 4.5 times as massive as the Earth and 1.6 times larger --
what planet enthusiasts call a super-Earth. Its mean density is close
to the density of the Earth, suggesting a possibly similar composition
as well.
The team discovered three additional longer-period planets in the
system from the HARPS-N radial velocities. In the inner regions, a
planet of 2.7 times the Earth's mass orbits HR 8832 in 6.8 days, and a
planet of 8.7 times the Earth is in a 46.8-day orbit. *If* those 2
planets are in a coplanar configuration with the innermost one, as
often occurs in compact systems, they too might be transiting.
Motivated by that prospect, future observations to observe the
potential transits have already been organized. The system includes
in addition a giant planet (of small-Saturn type) at 2.1 astronomical
units, orbiting the star in slightly more than 3 years. The system,
reminiscent of our own Solar System with inner small planets and the
outer gaseous one, is of obvious interest to the astronomical
community. Indeed, the proximity and brightness of the star makes the
system the most favourable one for characterisation of the planetary
physical properties. For atmospheric studies, astronomers are already
planning observations with ground-based high-resolution spectrographs
and with the future James Webb space telescope (JWST) using trans-
mission-spectroscopy techniques; during the transit some of the light
of the star passes through the atmosphere of the planet on its way to
the observer, and carries the spectral signature of the chemical
species present in the atmosphere.
BROWN DWARFS MORE LIKE PLANETS THAN STARS
California Institute of Technology
Brown dwarfs are relatively cool, dim objects that are difficult to
detect and hard to classify. They are too massive to be planets, yet
possess some planet-like characteristics; they are too small to
sustain hydrogen fusion reactions at their cores, a defining
characteristic of stars, yet they have star-like attributes. By
observing a brown dwarf 20 light-years away using both radio and
optical telescopes, astronomers have found another feature that makes
those so-called failed stars more like super-sized planets -- they
have powerful aurorae near their magnetic poles. Brown dwarfs are not
like small stars in terms of their magnetic activity; they are more
like giant planets with hugely powerful aurorae. In the early 2000s,
astronomers began finding that brown dwarfs emit radio waves. At
first, everyone assumed that the brown dwarfs were creating the radio
waves in basically the same way that stars do -- through the action of
an extremely hot atmosphere, or corona, heated by magnetic activity
near the object's surface. But brown dwarfs do not generate large
flares and charged-particle emissions in the way that our Sun and
other stars do, so the radio emissions were surprising. Brown dwarfs
can actually pulse at radio frequencies and we see a similar pulsing
phenomenon, due to aurorae, from planets in the Solar System. Auroral
displays result when charged particles, carried by the stellar wind
for example, manage to enter a planet's magnetosphere, the region
where such charged particles are influenced by the planet's magnetic
field. Once within the magnetosphere, the particles get accelerated
along the planet's magnetic-field lines to the planet's poles, where
they collide with gas atoms in the atmosphere and produce the bright
emissions associated with aurorae.
Astronomers conducted an extensive observation campaign of a brown
dwarf, called LSRJ 1835+3259, with the Very Large Array (VLA) as well
as optical instruments that included Palomar's Hale Telescope and the
Keck Observatory telescopes. With the VLA they detected a bright
pulse of radio waves that appeared as the brown dwarf rotated. The
object rotates every 2.84 hours, so the researchers were able to watch
nearly three full rotations over the course of a single night. Next,
the astronomers used the Hale Telescope to observe that the brown
dwarf varied optically with the same period as the radio pulses.
Observing the H-alpha emission line, they found that the object's
brightness varied periodically. Finally, they used the Keck
telescopes to measure precisely the brightness of the brown dwarf over
time to establish that the hydrogen emission is a signature of aurora
near the surface of the brown dwarf. As the electrons spiral down
toward the atmosphere, they produce radio emissions, and then when
they hit the atmosphere, they excite hydrogen in a process that occurs
at the Earth and other planets, though in the brown dwarf it is tens
of thousands of times more intense. We now know that that kind of
auroral behaviour extends all the way from planets up to brown dwarfs.
In the case of brown dwarfs, charged particles cannot be driven into
their magnetosphere by a stellar wind, as there is no stellar wind.
Some other source, such as an orbiting planet moving through the brown
dwarf's magnetosphere, may be generating a current and producing the
aurora. The coolest brown dwarfs have atmospheres somewhat similar to
what we would expect for many exo-planets, and it is possible actually
to look at a brown dwarf and study its atmosphere without having a
star nearby that is a factor of a million times brighter obscuring the
observations, as happens in the cases of exo-planets.
STARS IN MILKY WAY HAVE MOVED
New Mexico State University
Researchers using the Sloan Digital Sky Survey (SDSS) have created a
new map of the Milky Way that suggests that nearly a third of the
stars must dramatically have changed their Galactic orbits. The
discovery brings a new understanding of how stars are formed, and how
they travel through the Galaxy. To build a new map of the Milky Way,
scientists used the SDSS Apache Point Observatory Galactic Evolution
Explorer (APOGEE) spectrograph to observe 100,000 stars over a 4-year
interval. The key to creating and interpreting that map of the
Galaxy is measuring the elements in the atmosphere of each star.
From the chemical composition of a star, we can learn something of
its ancestry and life history. The chemical information comes from
spectra, which show that the chemical makeup of our Galaxy is
constantly changing. Stars create heavier elements in their cores,
and when those stars reach the end of their evolution the heavier
elements are returned into the gas from which the next stars form.
As a result of that process of 'chemical enrichment', each generation
of stars has a higher percentage of heavier elements than the
previous one did. In some regions of the Galaxy, star formation has
proceeded more vigorously than in other regions, and in those more
vigorous regions, more generations of new stars have formed. That
means that the average amount of heavier elements in stars varies
among different parts of the Galaxy. So astronomers can make an
educated gues as to what part of the Galaxy a star was born in by
tracing the amount of heavy elements in that star.
The team used APOGEE data to map the relative amounts of 15 elements,
including carbon, silicon, and iron, for stars all over the Galaxy.
What they found surprised them -- up to 30 per cent of stars had
compositions indicating that they were formed in parts of the Galaxy
far from their current positions. When the team looked at the pattern
of element abundances in detail, it found that many of the data
could be explained by a model in which stars migrate radially, moving
closer or farther from the Galactic Centre with time. Such motions
are referred to as 'migration' and may be caused by irregularities in
the Galactic disc, such as the spiral arms. Evidence of stellar
migration had previously been seen in stars near the Sun, but the new
study is the first clear evidence that migration occurs throughout the
Galaxy. The present results utilize only a small fraction of the
available APOGEE data. A more comprehensive analysis may elucidate
the chemistry and shape of our Galaxy much more clearly.
THE LARGEST FEATURE IN THE UNIVERSE
RAS
A team of astronomers has found what appears to be the largest feature
in the observable Universe: a ring of nine gamma ray bursts -- and
hence galaxies -- 5 billion light-years across. Gamma-ray bursts
(GRBs) are the most luminous events in the Universe, releasing as
much energy in a few seconds as the Sun does over its 10-billion-year
lifetime. They are thought to be the result of massive stars
collapsing into black holes. Their huge luminosity helps astronomers
to recogize the locations of distant galaxies, something the team
exploited. The GRBs that make up the newly discovered ring were
observed with a variety of space- and ground-based observatories.
They appear to be at very similar distances from us -- around 7
billion light-years -- in a circle 36 degrees across on the sky, or
more than 70 times the diameter of the Full Moon. That implies that
the ring is more than 5 billion light years across; there is said to
be only a 1 in 20,000 probability of the GRBs being in such a
distribution by chance.
Most current models indicate that the structure of the cosmos is
uniform on the largest scales. That 'Cosmological Principle' is
backed up by observations of the early Universe and its microwave
background signature, seen by the WMAP and Planck satellites. Other
recent results and this new discovery challenge the principle, which
sets a theoretical limit of 1.2 billion light-years for the largest
structures. The newly discovered ring is almost five times as large.
If the ring represents a real spatial structure, then it has to be
seen nearly face-on because of the small variations of GRB distances
around the object's centre. The ring could instead be a projection of
a sphere, where the GRBs all occurred within a 250-million-year
interval, short compared with the age of the Universe. A spheroidal
ring projection would mirror the strings of clusters of galaxies seen
to surround voids in the Universe; voids and string-like formations
are seen and predicted by many models of the cosmos. The newly
discovered ring is, however, at least ten times larger than known
voids. The team now wants to find out more about the ring, and to
establish whether the known processes for galaxy formation and large-
scale structure could have led to its creation, or if astronomers
need to revise their theories of the evolution of the cosmos.
DISTANT PROTO-GALAXY CONNECTED TO COSMIC WEB
California Institute of Technology
Astronomers have discovered a giant swirling disc of gas 10 billion
light-years away -- a galaxy-in-the-making that is actively being fed
cool primordial gas tracing back to the Big Bang. Using the Cosmic
Web Imager (CWI) at Palomar Observatory, the researchers imaged the
proto-galaxy and found that it is connected to a filament of the
intergalactic medium, the cosmic web made of diffuse gas that criss-
crosses between galaxies and extends throughout the Universe. The
finding provides the strongest observational support yet for what is
known as the cold-flow model of galaxy formation. That model holds
that in the early Universe, relatively cool gas funnelled down from
the cosmic web directly into galaxies, fuelling rapid star formation.
The protogalactic disc that the team has identified is about 400,000
light-years across -- about four times larger in diameter than the
Milky Way. It is situated in a system dominated by two quasars, the
closest of which, UM287, is positioned such that its emission is
beamed like a torch, helping to illuminate the cosmic-web filament
that is feeding gas into the spiralling proto-galaxy. Last year,
astronomers announced the discovery of what they thought was a large
filament next to UM287. The feature they observed was brighter than
it should have been if indeed it was only a filament. It seemed that
there must be something else there. In 2014 September, researchers
decided to follow up by observing the system with the CWI. As an
integral-field spectrograph, CWI allowed the team to collect images
around UM287 at hundreds of different wavelengths simultaneously,
revealing details of the system's composition, mass distribution, and
velocity. The team focused on a range of wavelengths around an
emission line in the ultraviolet known as the Lyman-alpha line.
That line, a fingerprint of atomic hydrogen gas, is commonly used by
astronomers as a tracer of primordial matter. The researchers
collected a series of spectral images that combined to form a
multiiwavelength map of a patch of sky around the two quasars.
That delineated areas where gas is emitting in the Lyman-alpha line,
and indicated the velocities with which that gas is moving with
respect to the centre of the system.
The images plainly show that there is a rotating disc -- you can see
that one side is moving closer to us and the other is moving away.
And you can also see that there is a filament that extends beyond the
disc. The measurements indicate that the disc is rotating at a rate
of about 400 km/s, somewhat faster than the Milky Way's own rate of
rotation. The filament has a more or less constant velocity. It is
basically funnelling gas into the disc at a fixed rate. Once the gas
merges with the disc inside the dark-matter halo, it is pulled round
by the rotating gas and dark matter in the halo. The new observations
and measurements provide the first direct confirmation of the
so-called cold-flow model of galaxy formation. Hotly debated since
2003, that model stands in contrast to the standard, older view of
galaxy formation. The standard model said that when dark-matter
haloes collapse, they pull a great deal of normal matter in the form
of gas along with them, heating it to extremely high temperatures.
The gas then cools very slowly, providing a steady but slow supply of
cold gas that can form stars in growing galaxies. That model seemed
fine until 1996, when a distant population of galaxies was discovered
producing stars at a very high rate only two billion years after the
Big Bang. The standard model cannot provide the prodigious fuel
supply for such rapidly forming galaxies. The cold-flow model
provided a potential solution. Theorists suggested that relatively
cool gas, delivered by filaments of the cosmic web, streams directly
into proto-galaxies. There, it can quickly condense to form stars.
Simulations show that as the gas falls in, it contains tremendous
amounts of angular momentum, or spin, and forms extended rotating
discs. That is a direct prediction of the cold-flow model, and is
exactly what we see -- an extended disc with lots of angular momentum
that we can measure.
LOST LITHIUM DESTROYED BY ANCIENT STARS
RAS
Lithium, the lightest metal, used in batteries and mood-stabilizing
drugs, is rarer than it should be. Models of the period after the Big
Bang explain how it, hydrogen and helium were synthesized in nuclear
reactions, before the Universe cooled enough for the stars and planets
that we see today to come into being. But some astronomers think that
about three times as much lithium ought to have been produced in that
earliest epoch as remains today in the oldest stars in the galaxy, and
the difference has proved hard to explain. Now a group of scientists
thinks it has the answer to that so-called 'lithium problem': it was
destroyed and re-accumulated by the stars shortly after they were
born. In the past astronomers have speculated on what might be
responsible for the lithium deficit. Ideas included as-yet-unknown
aspects of particle physics, nuclear physics or even new models of
cosmology. The team looked at how much lithium there would have been
when a particular sub-set of the first long-lived stars formed, just a
few hundred million years after the Big Bang. They still exist today,
and so provide astronomers with some insight into the history of the
Universe and how its composition has changed. The stars have between
50 and 85% of the mass of the Sun, have lifetimes that are
significantly longer, and are thought to remain stable on the main
sequence for between 15 and 30 billion years. They are poor in most
'metals', which in astronomy means every element heavier than helium.
The scientists modelled the way that those stars process lithium,
starting with the early part of their lives when they are still
contracting and heating up under the influence of gravity.
In that pre-main-sequence phase, the new model suggests that there is
more mixing in the different layers of those objects. To put that in
context, stars have a hot core, where nuclear fusion is converting
hydrogen to helium, a cooler outer layer where convection cycles
material from above the core to the surface and down again, and a
surface where radiation (including light and heat) escapes into space.
The new work indicates that in the first phase of their lives, the
low-mass stars have an extra mixing ('overshooting') at the base of
the convection zone, where surface lithium is brought to the hot
interior and almost completely destroyed. Pre-main-sequence stars are
also surrounded by clouds of the residual gas and dust from which they
formed. The cloud will over time be pulled on to the star, adding
lithium to its surface. As the star ages, the convective zone becomes
shallower, so material is no longer sent to the core, to some extent
offsetting the earlier destruction of lithium. Stars also shine
brightly in ultraviolet light, and the 'radiation pressure' of that
light eventually blows the disc materials away, stopping the star from
accumulating more lithium. The stars then enter the main sequence and
settle into a long period of stability. When we observe them now,
between 10 and 12 billion years later, they show a constant abundance
of lithium, which is about one third of the primordial level.
The work is a completely new approach to the lithium problem. The
model may not only explain the loss of lithium in stars, but could
also help to explain why the Sun has fifty times less lithium than
similar stars and why stars with planets have less lithium than stars
on their own. In the next decade new observatories like the European
Extremely Large Telescope under construction in Chile should allow
astronomers to look back at the first metal-poor stars as they formed,
and confirm the rapid loss of lithium in the early Universe.
Topic: Late August Astronomy Bulletin (Read 1215 times)